Strongs Concordance:

(Heb. pered), so called from the quick step of the animal or its power of carrying loads. It is not probable that the Hebrews bred mules, as this was strictly forbidden in the law (Leviticus 19:19), although their use was not forbidden. We find them in common use even by kings and nobles (2 Samuel 18:9; 1 Kings 1:33; 2 Kings 5:17; Psalms 32:9). They are not mentioned, however, till the time of David, for the word rendered "mules" (R.V. correctly, "hot springs") in Genesis 36:24 (yemim) properly denotes the warm springs of Callirhoe, on the eastern shore of the Dead Sea. In David's reign they became very common (2 Samuel 13:29; 1 Kings 10:25).

Mules are not mentioned in the New Testament. Perhaps they had by that time ceased to be used in Palestine.

a hybrid animal, the offspring of a horse and an ass. "The mule is smaller than the horse, and is a remarkably hardy, patient, obstinate, sure-footed animal, living, ordinarily, twice as long as a horse."

McClintock and Strong's Cyclopedia. It was forbidden to the Isr'lites to breed mules, but sometimes they imported them. It would appear that only kings and great men rode on mules. We do not read of mules at all in the New Testament; perhaps therefore they had ceased to be imported.

Webster's 1828 Dictionary

Mule

MULE, noun [Latin mulus.]

1. A quadruped of a mongrel breed, usually generated between an ass and a mare, sometimes between a horse and a she-ass. But the name is applied to any animal produced by a mixture of different species.

2. A plant or vegetable produced by impregnating the pistil of one species of plant with the farin or fecundating dust of another. This is called also a hybrid.